Among different DC/DC converters, an inrush current and an overshoot voltage appear in the startup period, which may damage the whole system, decrease the power efficiency, and cause system errors. Therefore, the soft-start function is used to smoothly increase the output voltage and the inductor current to the target values. There are numerous different types of soft-start circuits which have been implemented. The conventional soft-start circuit is shown in FIG. 1. In FIG. 1, the DC/DC converter 1 (which is a switching mode DC/DC converter) includes a mode switch circuit, a soft-start circuit, an error amplifier, a first comparator, a second comparator, an oscillator OSC, a driver, a switch sw, a diode, an inductor with an inductor current iL, a capacitor, a first resistor, and a second resistor. The switch sw is coupled with the inductor and the diode at a node SW, the DC/DC converter receives an input voltage VIN and generates an output voltage VO across two output terminals 12 and 13, and a voltage divider 14 comprises the first resistor and the second resistor, and generates a detected voltage Vfb of the output voltage VO to for provide a feedback to an input terminal 11 (of the error amplifier) of the DC/DC converter.
However, when the duty ratio=VO/VIN is too low (e.g., less than 10%) and the switching frequency is also very slow (e.g. 50 KHz), the inrush current and the overshoot voltage will be much higher than the safe range which the system can stand. When the duty ratio is too low and the switching frequency is also very slow, the waveforms of the inductor current iL, a detected value (Vfb) of the output voltage VO and an voltage VSW of a node SW of the DC/DC converter as shown in FIG. 1 are shown in FIG. 2. In the waveforms related to Vfb of FIG. 2, a trapezoidal waveform shows a safe range that the DC/DC converter can bear, and the waveform of Vfb exceeds the safe range at the moment of a little bit more than 500 μsec.
When the switching frequency is relatively fast (e.g. 1 MHz), the inrush current and the overshoot voltage still happen and are higher than the safe range that the DC/DC converter can bear. When the switching frequency is relatively fast, the waveforms of the inductor current iL, a detected value (Vfb) of the output voltage VO and an voltage VSW of a node SW of the DC/DC converter of FIG. 1 are shown in FIG. 3. In the waveforms related to Vfb of FIG. 3, a trapezoidal waveform shows a safe range that the DC/DC converter can bear, and the waveform of Vfb always exceeds the safe range, and begins to exceed the safe range dramatically at the moment of about 100 μsec.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived a methodology of on-chip soft-start circuits for a switching mode DC/DC converter.